* Drop support for PHP 5.
* Drop support for APC (APC was only available for PHP5 - It is the predecessor of APCu)
* Emit a warning and return null if igbinary_unserialize() is passed more data to unserialize than expected.
* Fix compilation against PHP 7.4-dev. Igbinary does NOT yet properly serialize/unserialize all classes with PHP 7.4-dev's typed properties.
* The serialization format is exactly the same as igbinary 2.x

* Be more aggressive about deduplication when generating serialization of arrays in php 7.0+.
* Define HAVE_IGBINARY on Unix/Linux. (previously defined only on Windows)
* Update formatting/wording of documentation.

- Fixes bug #126: Fatal error: "igbinary_serialize_zval: zval has unknown type 0" (IS_UNDEF)
Make this a warning instead of a fatal error (and serialize as null instead), since IS_UNDEF is a known type.
Later releases will fix the root cause of the warning, and consistently omit array/object/other entries for IS_UNDEF.

- Compatible with PHP 5.2 - 7.1
- Fixes crash in Memcached->setMulti (in php 7.0+) when the first level of array elements have references as values.
Other extensions using igbinary shouldn't be affected.